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58 lines
1.5 KiB
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58 lines
1.5 KiB
ReStructuredText
SPC water model
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===============
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The SPC water model specifies a 3-site rigid water molecule with
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charges and Lennard-Jones parameters assigned to each of the 3 atoms.
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In LAMMPS the :doc:`fix shake <fix_shake>` command can be used to hold
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the two O-H bonds and the H-O-H angle rigid. A bond style of
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*harmonic* and an angle style of *harmonic* or *charmm* should also be
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used.
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These are the additional parameters (in real units) to set for O and H
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atoms and the water molecule to run a rigid SPC model.
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| O mass = 15.9994
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| H mass = 1.008
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| O charge = -0.820
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| H charge = 0.410
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| LJ epsilon of OO = 0.1553
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| LJ sigma of OO = 3.166
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| LJ epsilon, sigma of OH, HH = 0.0
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| r0 of OH bond = 1.0
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| theta of HOH angle = 109.47
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Note that as originally proposed, the SPC model was run with a 9
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Angstrom cutoff for both LJ and Coulombic terms. It can also be used
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with long-range Coulombics (Ewald or PPPM in LAMMPS), without changing
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any of the parameters above, though it becomes a different model in
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that mode of usage.
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The SPC/E (extended) water model is the same, except
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the partial charge assignments change:
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| O charge = -0.8476
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| H charge = 0.4238
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See the :ref:`(Berendsen) <howto-Berendsen>` reference for more details on both
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the SPC and SPC/E models.
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Wikipedia also has a nice article on `water models <http://en.wikipedia.org/wiki/Water_model>`_.
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----------
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.. _howto-Berendsen:
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**(Berendsen)** Berendsen, Grigera, Straatsma, J Phys Chem, 91,
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6269-6271 (1987).
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Commands_all.html
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